This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
1. Field of the Invention
The present invention relates to the field of offshore drilling technology. More specifically, the present invention relates to a floating marine drilling unit that employs a riser and mooring system suitable for use in icy arctic waters.
2. Discussion Of Technology
As the world's demand for fossil fuels increases, energy companies find themselves pursuing hydrocarbon resources located in more remote and hostile areas of the world, both onshore and offshore. Such areas include Arctic regions where ambient air temperatures reach well below the freezing point of water. Specific onshore examples include Canada, Greenland and northern Alaska.
One of the major problems encountered in offshore arctic regions is the continuous formation of sheets of ice on the water surface. Ice masses formed off of coastlines over water depths greater than 20 or 25 meters are dynamic in that they are almost constantly moving. The ice masses, or ice sheets, move in response to such environmental factors as wind, waves, and currents. Ice sheets may move laterally through the water at rates as high as about a meter/second. Such dynamic masses of ice can exert enormous forces on structural objects in their path. Therefore, offshore structures operating in arctic seas must be able to withstand or overcome the forces created by moving ice.
Another danger encountered in arctic waters is pressure ridges of ice. These are large mounds of ice which usually form within ice sheets and which may consist of overlapping layers of sheet ice and re-frozen rubble caused by the collision of ice sheets. Pressure ridges can be up to 30 meters thick or more and can, therefore, exert proportionately greater forces than ordinary sheet ice.
Bottom supported stationary structures are particularly vulnerable in offshore arctic regions, especially in areas of deep water. The major force of an ice sheet or pressure ridge is directed near the surface of the water. If an offshore structure comprises a drilling platform or deck supported by a long, comparatively slender column which extends well below the surface, the bending moments caused by the laterally moving ice may well be sufficient to topple the platform.
U.S. Pat. No. 4,048,943, issued in 1977 to Gerwick, proposed a drilling unit having an inverted, conically-shaped structure floating generally above the water line. The inverted structure includes a top surface or deck for supporting drilling equipment and activities. The drilling unit also includes a large, cylindrical caisson floating below the inverted conically-shaped structure. The caisson then includes a radially tapered upper portion, preferably conically shaped, connected to the inverted, conically-shaped structure below the water line. Mooring lines are attached to the caisson and then anchored to the sea floor to secure the drilling unit's position in the water.
The drilling unit of Gerwick includes means for vertically reciprocating the caisson. In this way, the upper portion of the caisson can obliquely contact ice sheets and other ice masses with sufficient dynamic force to pierce and break the ice. The moving ice strikes the slanted wall of the cone-shaped structure, and is uplifted. The uplift of the ice not only tends to break the ice, but also substantially alleviates the horizontal crushing force of the ice on the structure.
Other drilling structures having inverted, conical-shaped hulls are disclosed in U.S. Pat. No. 3,766,874 issued to Helm, et al. and U.S. Pat. No. 4,434,741 issued to Wright, et al. Such structures employ hulls that are generally frusto-conical in shape to fracture ice impinging on the hull. The hulls are moored to the sea bottom using traditional chains or wire ropes.
In traditional offshore operations, the use of chains, wire ropes or synthetic ropes for mooring lines is desirable. These mooring lines offer flexibility to the floating structure, allowing the structure to move in response to waves, wind, and currents. At the same time, such traditional mooring lines may not provide sufficient strength to withstand the high shear forces presented by moving ice sheets. Current mooring systems on floating vessels have limited capability to resist ice loads and are generally limited to open water and warm-weather seasonal drilling or production operations.
Full development of offshore oil and gas fields requires operations from a given location; for example, the drilling of multiple wells from a given location. This is true even in arctic locations where ice sheets cover the water much of the year. It is desirable to maintain year-round operations to avoid the expense of seasonal relocation and the complexities of multi-year re-entry in partially drilled wells.
Therefore, a need exists for an improved mooring system that is capable of maintaining an offshore floating unit on a given location in an arctic environment.